The present invention relates to a primer for .alpha.-cyanoacrylate-based resin compositions for use in bonding non-polar or highly crystallized resins such as polyolefins, polyethylene terephthalates, nylons, fluorine-containing resins, soft PVC films, etc., or in coating and printing such resins or other applications, and more particularly, to a primer for .alpha.-cyanoacrylate resin compositions, which can exhibit a high bonding strength by a simple procedure. 2. DESCRIPTION OF THE PRIOR ART
The above non-polar or highly crystallized resins are extremely useful for sheets or molded products used in automobiles, electric appliances, etc., and can find many utilizations, because they have a light weight and excellent properties such as physical properties and a non-corrosive property. With such molded products or the like, however, the following problem is encountered: In fixing them to other parts or in coating their surfaces for use, bonding or coating is difficult, because they are non-polar or highly crystallized resins as described above.
Thereupon, several approaches to improve an adhesion property or the like have been proposed. For example:
(a) A method for partially oxidizing and activating difficultly-bondable materials such as polyethylenes and polypropylenes by a flame-treatment, a corona discharge, an irradiation or another treatment thereof;
(b) The use, as an undercoating, of a primer comprising a polyethylene chloride, a polypropylene chloride or a fatty acid-modified alkyd resin dissolved in an organic solvent (see Japanese Patent Application Laid-open Nos. 98062/77; 141328/81; and 119929/82); and
(c) The use, as an undercoating, of a primer comprising an organic metallic compound in association with an .alpha.-cyanoacrylate-based adhesive (see Japanese Patent Application No. 203684/85).
With such a method for oxidizing and activating the difficultyly-bondable material by the flame treatment or another treatment thereof, as described in the above item (a), however, a special treating system and process are required and particularly, when a uniform treatment is not performed for a material of a complicated configuration which is to be bonded, an operation is troublesome and a lining cost is increased, resulting in a decreased economy.
With the primer described in the item (b) and employing polyethylene chloride or the like, there are pointed out disadvantages that even though an adherability thereof to polyolefins is good, an adherability to final coatings is poor, and even when such primer exhibits a good adherability to final coatings and polyolefins, an adherability between films is reduced as a result of repeating of a cold cycle (see Japanese Patent Application Laid-open No. 8734/83).
Further, the primer described in the item (c) has an adhesion property as high as a fracture of material to molded products of polypropylene, but have an insufficient bonding strength to molded products of polyethylene and have not a strength as high as a fracture of material. In addition, resistances to water and humidity at a joint formed between bonded materials are being improved, but are still insufficient, and a sufficient bonding strength cannot be satisfactorily maintained.